about_Functions

04/20/2010

10 minutes to read

In this article

Applies To: Windows PowerShell 2.0

TOPIC
about_Functions
SHORT DESCRIPTION
Describes how to create and use functions in Windows PowerShell.
LONG DESCRIPTION
A function is a list of Windows PowerShell statements that has a name
that you assign. When you run a function, you type the function name.
The statements in the list run as if you had typed them at the command
prompt.
Functions can be as simple as:
function Get-PowerShellProcess {get-process powershell}
or as complex as a cmdlet or an application program.
Like cmdlets, functions can have parameters. The parameters can be named,
positional, switch, or dynamic parameters. Function parameters can be read
from the command line or from the pipeline.
Functions can return values that can be displayed, assigned to variables,
or passed to other functions or cmdlets.
The function's statement list can contain different types of statement
lists with the keywords Begin, Process, and End. These statement lists
handle input from the pipeline differently.
A filter is a special kind of function that uses the Filter keyword.
Functions can also act like cmdlets. You can create a function that works
just like a cmdlet without using C# programming. For more information,
see about_Functions_Advanced.
Syntax
The following is the syntax for a function:
function [<scope:>]<name> [([type]$parameter1[,[type]$parameter2])]
{
param([type]$parameter1 [,[type]$parameter2])
dynamicparam {<statement list>}
begin {<statement list>}
process {<statement list>}
end {<statement list>}
}
A function includes the following items:
- A Function keyword
- A scope (optional)
- A name that you select
- Any number of named parameters (optional)
- One or more Windows PowerShell commands enclosed in braces ({})
For more information about the Dynamicparam keyword and dynamic
parameters in functions, see about_Functions_Advanced_Parameters.
Simple Functions
Functions do not have to be complicated to be useful. The simplest
functions have the following format:
function <function-name> {statements}
For example, the following function starts Windows PowerShell with
the Run as Administrator option.
function Start-PSAdmin {start-process powershell -verb RunAs}
To use the function, just type "Start-PSAdmin".
To add statements to the function, use a semi-colon (;) to separate the
statements, or type each statement on a separate line.
For example, the following function finds all .jpg files in the current
user's directories that were changed after the start date.
function Get-NewPix
{
$start = get-date -month 1 -day 1 -year 2010
$allpix = get-childitem -path $env:userprofile\* -include *.jpg -recurse
$allpix | where {$_.lastwritetime -gt $start}
}
You can create a toolbox of useful small functions. Add these functions
to your Windows PowerShell profile, as described in about_Profiles and
later in this topic.
Function Names
You can assign any name to a function, but functions that you share with
others should follow the naming rules that have been established for
all Windows PowerShell commands.
Functions names should consist of a verb-noun pair in which the verb
identifies the action that the function performs and the noun identifies
the item on which the cmdlet performs its action.
Functions should use the standard verbs that have been approved for all
Windows PowerShell commands. These verbs help us to keep our command names
simple, consistent, and easy for users to understand.
For more information about the standard Windows PowerShell verbs, see
"Cmdlet Verbs" on MSDN at http://go.microsoft.com/fwlink/?LinkID=160773.
Functions with Parameters
You can use parameters with functions, including named parameters,
positional parameters, switch parameters, and dynamic parameters. For
more information about dynamic parameters in functions,
see about_Functions_Advanced_Parameters.
Named Parameters
You can define any number of named parameters. You can include a default
value for named parameters, as described later in this topic.
You can define parameters inside the braces using the Param keyword, as
shown in the following sample syntax:
function <name> {
param ([type]$parameter1[,[type]$parameter2])
<statement list>
}
You can also define parameters outside the braces without the Param
keyword, as shown in the following sample syntax:
function <name> [([type]$parameter1[,[type]$parameter2])] {
<statement list>
}
There is no difference between these two methods. Use the method that
you prefer.
When you run the function, the value you supply for a parameter is
assigned to a variable that contains the parameter name. The value of
that variable can be used in the function.
The following example is a function called Get-SmallFiles. This function
has a $size parameter. The function displays all the files that are
smaller than the value of the $size parameter, and it excludes
directories:
function Get-SmallFiles {
param ($size)
Get-ChildItem c:\ | where {$._.length -lt $size -and !$_.PSIsContainer}
}
In the function, you can use the $size variable, which is the name
defined for the parameter.
To use this function, type the following command:
C:\PS> function Get-SmallFiles –size 50
You can also enter a value for a named parameter without the parameter
name. For example, the following command gives the same result as a
command that names the Size parameter:
C:\PS> function Get-SmallFiles 50
To define a default value for a parameter, type an equal sign and the
value after the parameter name, as shown in the following variation of
the Get-SmallFiles example:
function Get-SmallFiles ($size = 100) {
Get-ChildItem c:\ | where {$_.length -lt $size -and !$_.PSIsContainer}
}
If you type "Get-SmallFiles" without a value, the function assigns 100 to
$size. If you provide a value, the function uses that value.
Positional Parameters
A positional parameter is a parameter without a parameter name. Windows
PowerShell uses the parameter value order to associate each parameter
value with a parameter in the function.
When you use positional parameters, type one or more values after the
function name. Positional parameter values are assigned to the $args
array variable. The value that follows the function name is assigned to
the first position in the $args array, $args[0].
The following Get-Extension function adds the .txt file name extension to a
file name that you supply:
function Get-Extension {
$name = $args[0] + ".txt"
$name
}
C:\PS> Get-Extension myTextFile
myTextFile.txt
Switch Parameters
A switch is a parameter that does not require a value. Instead, you type
the function name followed by the name of the switch parameter.
To define a switch parameter, specify the type [switch] before the
parameter name, as shown in the following example:
function switch-Item {
param ([switch]$on)
if ($on) { "Switch on" }
else { "Switch off" }
}
When you type the On switch parameter after the function name, the
function displays "Switch on". Without the switch parameter, it displays
"Switch off".
C:\PS> Switch-Item -on
Switch on
C:\PS> Switch-Item
Switch off
You can also assign a Boolean value to a switch when you run the
function, as shown in the following example:
C:\PS> Switch-Item -on:$true
Switch on
C:\PS> Switch-Item -on:$false
Switch off
Piping Objects to Functions
Any function can take input from the pipeline. You can control how a
function processes input from the pipeline using Begin, Process, and End
keywords. The following sample syntax shows the three keywords:
function <name> {
begin {<statement list>}
process {<statement list>}
end {<statement list>}
}
The Begin statement list runs one time only, at the beginning of
the function.
The Process statement list runs one time for each object in the pipeline.
While the Process block is running, each pipeline object is assigned to
the $_ automatic variable, one pipeline object at a time.
After the function receives all the objects in the pipeline, the End
statement list runs one time. If no Begin, Process, or End keywords are
used, all the statements are treated like an End statement list.
The following function uses the Process keyword. The function displays
examples from the pipeline:
function Get-Pipeline
{
process {"The value is: $_"}
}
To demonstrate this function, enter an list of numbers separated by
commas, as shown in the following example:
C:\PS> 1,2,4 | Get-Pipeline
The value is: 1
The value is: 2
The value is: 4
When you use a function in a pipeline, the objects piped to the function
are assigned to the $input automatic variable. The function runs
statements with the Begin keyword before any objects come from the
pipeline. The function runs statements with the End keyword after all
the objects have been received from the pipeline.
The following example shows the $input automatic variable with Begin and
End keywords.
function Get-PipelineBeginEnd
{
begin {"Begin: The input is $input"}
end {"End: The input is $input" }
}
If this function is run by using the pipeline, it displays the following
results:
C:\PS> 1,2,4 | Get-PipelineBeginEnd
Begin: The input is
End: The input is 1 2 4
When the Begin statement runs, the function does not have the input from
the pipeline. The End statement runs after the function has the values.
If the function has a Process keyword, the function reads the data in
$input. The following example has a Process statement list:
function Get-PipelineInput
{
process {"Processing: $_ " }
end {"End: The input is: $input" }
}
In this example, each object that is piped to the function is sent to the
Process statement list. The Process statements run on each object, one
object at a time. The $input automatic variable is empty when the
function reaches the End keyword.
C:\PS> 1,2,4 | Get-PipelineInput
Processing: 1
Processing: 2
Processing: 4
End: The input is:
Filters
A filter is a type of function that runs on each object in the pipeline.
A filter resembles a function with all its statements in a Process block.
The syntax of a filter is as follows:
filter [<scope:>]<name> {<statement list>}
The following filter takes log entries from the pipeline and then
displays either the whole entry or only the message portion of the entry:
filter Get-ErrorLog ([switch]$message)
{
if ($message) { out-host -inputobject $_.Message }
else { $_ }
}
Function Scope
A function exists in the scope in which it was created.
If a function is part of a script, the function is available to
statements within that script. By default, a function in a script is not
available at the command prompt.
You can specify the scope of a function. For example, the function is
added to the global scope in the following example:
function global:Get-DependentSvs { get-service |
where {$_.dependentservices} }
When a function is in the global scope, you can use the function in
scripts, in functions, and at the command line.
Functions normally create a scope. The items created in a function, such
as variables, exist only in the function scope.
For more information about scope in Windows PowerShell, see about_Scope.
Finding and Managing Functions Using the Function: Drive
All the functions and filters in Windows PowerShell are automatically
stored in the Function: drive. This drive is exposed by the Windows
PowerShell Function provider.
When referring to the Function: drive, type a colon after Function, just
as you would do when referencing the C or D drive of a computer.
The following command displays all the functions in the current session
of Windows PowerShell:
get-childitem function:
The commands in the function are stored as a script block in the
definition property of the function. For example, to display the
commands in the Help function that comes with Windows PowerShell, type:
(get-childitem function:help).definition
For more information about the Function: drive, see Function.
Reusing Functions in New Sessions
When you type a function at the Windows PowerShell command prompt, the
function becomes part of the current session. It is available until the
session ends.
To use your function in all Windows PowerShell sessions, add the function
to your Windows PowerShell profile. For more information about profiles,
see about_Profiles.
You can also save your function in a Windows PowerShell script file.
Type your function in a text file, and then save the file with the .ps1
file name extension.
Writing Help for Functions
The Get-Help cmdlet gets help for functions, as well as for cmdlets,
providers, and scripts. To get help for a function, type Get-Help followed
by the function name.
For example, to get help for the Get-MyDisks function, type:
get-help Get-MyDisks
You can write help for a function by using either of the two following methods:
-- Comment-Based Help for Functions
Create a help topic by using special keywords in the comments. To create
comment-based help for a function, the comments must be placed at the
beginning or end of the function body or on the lines preceding the function
keyword. For more information about comment-based help, see
about_Comment_Based_Help.
-- XML-Based Help for Functions
Create an XML-based help topic, such as the type that is typically
created for cmdlets. XML-based help is required if you are localizing
help topics into multiple languages.
To associate the function with the XML-based help topic, use the
.ExternalHelp help comment keyword. For more information about the
ExternalHelp keyword, see about_Comment_Based_Help. For more information
about XML-based help, see "How to Write Cmdlet Help" in MSDN.
SEE ALSO
about_Automatic_Variables
about_Comment_Based_Help
about_Functions_Advanced
about_Functions_CmdletBindingAttribute
about_Parameters
about_Profiles
about_Scopes
about_Script_Blocks
Function (provider)